Does anyone have a good rule for converting to dictionary (u)form from masu form regarding the different verb groups? Most websites start with dictionary and go to masu but I learned starting with masu.

The trick of course is knowing what sort of verb is it. Generally, verbs whose final stem syllable (before masu) is an 'e' sound are ichidan (tabemasu, akemasu). Howeverm, there others (dekimasu, karimasu). So, you will have to learn it for each verb. However, it's quite easy to form the dictionary form.

As James said, without knowing the group of the verb, you can't really guess. Note that this is also true for the inverse step : i.e. you cannot, simply from the dictionary form guess what is the ます-form nor the て-form.

From there, look for go-dan (五段) or ichi-dan (一段) in your dictionary. During my studies, my teacher also reffered to them as "strong" and "weak" verbs respectively.

Then, it is pretty simple :

For godan:

Remove the ます stem

Take the last syllable of your verb, before the ます stem.

Change it to the "u" sound of the same line. (Be careful, the "u" sound of the "t" line is つ and not テゥ !).

For ichidan:

Remove the ます stem

Add る

So now, the real question is, in which category does your verb fall in ?

If the last syllabe of the dictionary form (a.k.a. the one just before your ます stem) is a "e" syllabe, it is ichidan. See for example, 食｛た｝べる, 教{おし}える, 負｛ま｝ける...

If your verb has only one syllabe (excluding the ます stem) it is also ichidan ! For example 見｛み｝る/見ます、着{き}る/着ます. Be careful, some of this verbs have false friends : they have the same dictionary form but are actually godan. Check for example 着る and 切{き}る/切ります.

If your verb ends with an i vowel, it is most often a godan, see 飲｛の｝む/飲みます, 書｛か｝く/書きます, 勝{か}つ/勝ちます. However, you really need to memorize the exceptions to this rule : some are ichidan, like 起｛お｝きる/起きます, 降｛お｝りる/降ります, 借｛か｝りる/借ります

There are a few exceptions to this rule, most notably :

する/します

来｛く｝る/来｛き｝ます

As a side note, when you will know about more forms, you can always deduce the group of a verb from two forms, and from there construct the other ones. godan verbs change at all forms (five, hence godan) and add a suffix while ichidan never change their root and are simply added a suffix (hence a one step verb).

For 五段 verbs you remove -imasu and add *-u", for 一段 verbs you remove -masu and add -ru. From the above table, as a best guess you can assume that most verbs with a masu-form on -imasu are 五段.

However, verbs ending on -jimasu are always 一段, and verbs ending on -simasu almost always 五段.

Furthermore, you should exert some caution for verbs ending on -nimasu and -bimasu, as there are not significantly more verbs of one type than another.

Finally, if you are able to identify the compounds of a compound verb and apply the rule regarding the number of syllables (one syllable before -masu must be 一段, except for 来ます and します), you will only need to remember a handful of exceptions.

For reference, there are about 3k verbs ending on -emasu, but they are always 一段, thus you remove -masu and add -ru.

A word of warning about these numbers: they do no include the frequencies with which each verb occurs. There might be many more 五段 verbs ending on -gimasu than 一段 ones, but most 五段 might be used only rarely.